1. Field of the Invention
The present invention relates to a variable-frequency resonance circuit using inductors and a variable capacitor and to a variable-frequency filter including a plurality of variable-frequency resonance circuits.
2. Description of the Related Art
Various types of resonance circuits for setting a pass band and an attenuation band by using a resonant frequency of an inductor and that of a capacitor have been proposed. Various types of high-frequency filters including a plurality of such resonance circuits have also been proposed. As a resonance circuit or a high-frequency filter of this type, various variable-frequency resonance circuits and variable-frequency filters that can adjust their characteristics have been proposed. Examples of the characteristics of a resonance circuit or a high-frequency filter are transmission characteristics, attenuation characteristics, and insertion loss.
A variable-frequency resonance circuit or a variable-frequency filter includes a variable capacitor, and by changing the capacitance of the variable capacitor, the characteristics are adjusted.
For example, Japanese Unexamined Patent Application Publication No. 9-181538 discloses a variable-frequency filter shown in FIG. 12. FIG. 12 is a circuit diagram of such a variable-frequency filter 10P. The variable-frequency filter 10P includes first and second LC parallel resonance circuits that each include an inductor 20P, a variable capacitor 31P, and a capacitor 32P. These first and second LC parallel resonance circuits are connected between a ground and a transmission line for connecting first and second input/output terminals P1 and P2. A capacitor 401 is connected between the first input/output terminal P1 and a node between the transmission line and the first LC parallel resonance circuit. A capacitor 402 is connected between the second input/output terminal P2 and a node between the transmission line and the second LC parallel resonance circuit. A coupling capacitor 410 is connected between the node between the transmission line and the first LC parallel resonance circuit and the node between the transmission line and the second LC parallel resonance circuit.
In the above-described variable-frequency filter 10P, by changing the capacitance of the variable capacitors 31P of the first and second LC parallel resonance circuits, the frequency of a pass band or that of an attenuation band is adjusted.
However, the above-described variable-frequency filter 10P presents the following problem. FIGS. 13A and 13B are diagrams illustrating the transmission characteristics of the variable-frequency filter 10P. In FIGS. 13A and 13B, the solid line indicates the transmission characteristics exhibited when a substantially 400-MHz band is set as the pass band, while the broken line indicates the transmission characteristics exhibited when a substantially 520-MHz band is set as the pass band. In FIGS. 13A and 13B, BWf1p indicates the pass band width when a substantially 400-MHz band is set as the pass band, while BWf2p indicates the pass band width when a substantially 520-MHz band is set as the pass band. BWf1p′ indicates an imaginary pass band when the pass band width BWf1p is superimposed on the pass band width BWf2p. 
As shown in FIGS. 13A and 13B, the pass band widths BWf1p and BWf1p′ are wider than the pass band width BWf2p. 
In this manner, in the variable-frequency filter 10P having the above-described circuit configuration, the pass band width varies depending on a set frequency pass band.